HVDC power transmission has become increasingly important due to increasing need for power supply or delivery and interconnected power transmission and distribution systems. Power systems such as electrical power distribution or transmission systems generally include a protection system for protecting, monitoring and controlling the operation and/or functionality of other components included in the power system, which other components hence may be referred to as protected units. Such protection systems may for example be able to detect short circuits, overcurrents and overvoltages in power lines, transformers and/or other parts or components of the power system. The protection systems can include protection equipment such as circuit breakers for isolating any possible faults for example occurring in power transmission and distribution lines by opening or tripping the circuit breakers. After the fault has been cleared, e.g. by performing repairs and/or maintenance on the component in which the fault has been detected, the power flow can be restored by closing the circuit breakers.
Interface arrangements are known to be connected between an AC power system and a DC power system. Such an arrangement typically includes a converter, such as a voltage source converter, for conversion of AC power to DC power, or vice versa. The interface arrangement has a DC side for coupling to the DC power system and an AC side for coupling to the AC power system. The arrangement often includes a transformer having a primary side connected to the AC system and a secondary side for coupling to the converter.
For example in a HVDC power system, there is generally included an interface arrangement including or constituting an HVDC converter station, which is a type of station configured to convert high voltage DC to AC, or vice versa. An HVDC converter station may comprise a plurality of elements such as the converter itself (or a plurality of converters connected in series or in parallel), one or more transformers, capacitors, filters, and/or other auxiliary elements. Converters may comprise a plurality of solid-state based devices such as semiconductor devices and may be categorized as line-commutated converters or voltage source converters, e.g. depending on the type of switches (or switching devices) which are employed in the converter. A plurality of solid-state semiconductor devices such as IGBTs may be connected together, for instance in series, to form a building block, or cell, of an HVDC converter.
For example with respect to HVDC converters for offshore applications, one concern may be the volume of the converter station, which is generally desired to be as small as possible. A reduction of the volume of the converter station will generally entail a reduction in the cost of the converter station and also for example in the cost of the offshore platform. Modular Multi-level Converters (MMCs) usually have a relatively small volume, or ‘footprint’ as compared to classic converters, since MMCs in general require no AC filters or DC passive filters, which usually have a relatively large volume or footprint. An MMC may for example include Cascaded Two Level (CTL) converter cells as building blocks. The converter volume is directly proportional to number of converter cells. Each CTL converter cell usually comprises two IGBTs as switches or switching devices and an electrical energy storage element. Electric energy storage elements such as capacitors in general have a relatively large volume or footprint. One possibility for reducing converter volume is reducing the number of converter cells.